This invention relates to encoders and to decoders for highly-correlated information signals and, more particularly, to such encoders and decoders for use in connection with a single channel, NTSC-compatible, widescreen enhanced-definition television (EDTV) system.
As is known, an original widescreen signal, comprised of a center panel and left and right sidepanels, has its center panel time-expanded and its left and right sidepanels time-compressed prior to such signal being broadcast as an NTSC compatible 4.2 MHz baseband signal to both widescreen receivers and standard NTSC receivers. When received by a widescreen receiver (i.e., one displaying a picture having an aspect ratio such as 2:1, 16:9 or 5:3), the time-expanded center panel is compressed to its original size and the time-compressed sidepanels are expanded to their original size before picture display takes place (thereby reproducing the entire original widescreen picture on the screen of the widescreen receiver). The use of signal compression techniques for the sidepanels of the picture takes advantage of the horizontal overscan region of a standard NTSC television receiver display, so that such a standard NTSC receiver displays only the time-expanded center panel on its standard 4:3 aspect ratio screen (the time-compressed sidepanels being hidden due to the horizontal overscan).
A single channel NTSC compatible, widescreen EDTV television signal includes more information than is normally included in a conventional NTSC 4.2 MHz baseband television signal. A conventional NTSC signal includes luma information in a frequency band up to 4.2 MHz and chroma information in a more limited band which modulates a 3.58 MHz sub-carrier. A single channel, NTSC compatible, widescreen EDTV signal includes both high-frequency luma information in a band above 4.2 MHz. and sidepanel information, in addition to the luma and chroma information of a conventional NTSC signal. Ideally, this additional information should be encoded in manner such that it can be decoded at a widescreen receiver without any crosstalk taking place between the different types of encoded information, and without causing any degradation of the picture displayed by a standard NTSC receiver due to the presence of such encoded information.
Reference is now made to co-pending application Ser. No. 07/139,338, filed Dec. 29, 1987 by Isnardi et al., and assigned to the same assignee as the present application. This application discloses a single channel, NTSC compatible, widescreen EDTV system in which the original widescreen signal is comprised of high-frequency luma and sidepanel components, in addition to a main component comprised of the time-expanded center panel and time-compressed sidepanel low frequencies. Each of these three components is separately intraframe averaged. Intraframe averaging involves averaging the pixels values of each pair of neighboring image pixels in the vertical-temporal plane defined by the two interlaced fields of each NTSC frame. Such intraframe averaging significantly reduces the image data that need be transmitted, without introducing any significant error, since the image data defined by such a pair of neighboring pixels is almost nearly always highly correlated in any single frame. The intraframe-averaged high-frequency sidepanel and luma components quadrature-modulate a sub-carrier, which quadrature-modulated sub-carrier is then added to the intraframe-averaged main component, thereby providing an NTSC compatible 4.2 MHz baseband signal.
The use in the Isnardi et al. application of intraframe averaging allows perfect separation (i.e., no crosstalk) in the vertical-temporal plane of the main component and each of the two quadrature-modulated components by the decoder in the widescreen receiver. However, the main component in Isnardi et al. includes both the luma and chroma portions of a standard NTSC signal. Intraframe averaging does not allow for separation of luma and chroma in the widescreen receiver. They need to be separated therein by some other means, such as by linear, time-invariant, vertical-temporal filtering of luma and chroma. If such filters were ideal, no crosstalk would take place. However, in practice, no such filter is ideal. Therefore, significant unwanted crosstalk does take place between the luma and chroma portions of the main component and the other additional information components. Furthermore, luma, time-invariant, vertical-temporal filtering and intraframe averaging do not co-exist synergistically: they tend to fight each other. Furthermore, as single channel, NTSC compatible, widescreen EDTV system development continues, it becomes apparent that more and more additional information components need be included in the television signal transmitted to both widescreen receivers and standard NTSC receivers. This means that the information contained in the luma and chroma must be reduced even more than it is reduced by intraframe averaging, but still without any great detriment to the picture displayed by either the widescreen receiver or by the standard NTSC receiver. The quadruplex encoding and decoding technique of the present invention permits a single channel, NTSC compatible, widescreen EDTV system to transmit a large number of information components, including both luma and chroma, to both widescreen and standard NTSC receivers in a manner which permits the information to be separated into its various components by the decoder of each widescreen receiver without any significant amount of crosstalk between the various information components taking place, and without any significant degradation of the picture quality displayed by standard NTSC receivers.